The present invention relates to a reverse conducting thyristor, and more particularly, to a reverse conducting thyristor in which a gate turnoff thyristor and a diode are connected in reverse parallel to each other.
In general, in a reverse conducting thyristor, a gate turnoff thyristor (hereinafter referred to a xe2x80x9cGTO thyristorxe2x80x9d) and a free wheel diode are connected in reverse parallel to each other. FIG. 8 is a cross sectional view of a conventional reverse conducting thyristor generally indicated by the reference numeral 500. The reverse conducting thyristor comprises a diode portion denoted at A in FIG. 8, a GTO thyristor portion denoted at B in FIG. 8, and a separation portion denoted at C sandwiched between these two portions.
In this reverse conducting thyristor, a p layer 502 whose film thickness is about 90 xcexcm is formed on a first major surface of an Nxe2x88x92 silicon substrate 501 with the first major surface and a second major surface. For electrical separation between the diode portion A and the GTO thyristor portion B, the p layer 502 of the separation portion C is etched in the form of a groove, about 60 xcexcm in depth and about 5 mm in width. This makes a resistance value between the diode portion A and the GTO thyristor portion B about 300 through 500xcexa9. An n layer 503 is further formed on the p layer 502 in the GTO thyristor portion B.
On the other hand, an n+ layer 504 is formed on a second major surface of the nxe2x88x92 silicon substrate 501, and a p layer 505 and an n++ layer 506 are formed on the n+ layer 504.
Further, a cathode electrode 510 is disposed on the n layer 503 in the GTO thyristor portion B, and a gate electrode 511 is disposed on the p layer 502. In addition, a cathode electrode 512 is disposed on the p layer 502 in the diode portion A.
Meanwhile, an anode electrode 513 is disposed on the second major surface of the nxe2x88x92 silicon semiconductor substrate 501, as a common electrode for the diode portion A and the GTO thyristor portion B.
FIG. 9 is a circuitry diagram of the reverse conducting thyristor 500. The p layer 502 and the n+ layer 504 shown in FIG. 8 form the diode portion, while the n layer 503, the p layer 502, the n+ layer 504 and the p layer 505 form the GTO thyristor portion.
However, in the reverse conducting thyristor 500, a surface area size of the separation portion C separating the diode portion A from the GTO thyristor portion B is large, which is an obstacle against a size reduction of the reverse conducting thyristor 500. In addition, when a plurality of reverse conducting thyristors 500 are to be fabricated on a large wafer, the p layers 502 are etched unevenly in terms of depth within the wafer, and therefore, insulation characteristics of the separation portions C are not uniform.
It is an object of the present invention to provide a reverse conducting thyristor wherein a separation portion is small in surface area size and insulation characteristics is uniform.
The present invention is directed to a reverse conducting thyristor in which a diode and a thyristor are reverse parallel-connected and formed on the same substrate, comprising: a semiconductor substrate of a first conductivity type; a diode region of a second conductivity type of said diode, formed in a major surface of said semiconductor substrate; and a base region of the second conductivity type of said thyristor, formed in said major surface of said semiconductor substrate so as to be separated from said diode region of the second conductivity type by a separation region, wherein said separation region includes a thin film region of the second conductivity type formed in said major surface of said semiconductor substrate and a guard ring region of the second conductivity type.
In this reverse conducting thyristor, the separation portion has a small surface area size and variations in insulation characteristics at the separation portion are reduced. Further, with the thin film region formed in the separation portion, it is possible to prevent destruction of the element due to concentration of a leak current.
A distance between said guard ring region and said diode region of the second conductivity type and a distance between said guard ring region and said base region of the second conductivity type are both preferably 30 xcexcm or smaller. This is for increasing the breakdown voltage of the reverse conducting thyristor.
The depth of said guard ring region is preferably smaller than the depth of said diode region of the second conductivity type and the depth of said base region of the second conductivity type.
It is preferable that two or more such guard ring regions are formed. This is for obtaining sufficient insulation characteristics at the separation portion.
A distance between said guard ring regions is preferably 30 xcexcm or smaller. This is for increasing the breakdown voltage of the reverse conducting thyristor.
The depth of said thin film region is preferably 10 xcexcm or smaller. This is for obtaining sufficient insulation characteristics at the separation portion.
A concentration of an impurity of the second conductivity type contained in said thin film region is preferably lower than concentrations of impurities of the second conductivity type contained in said diode region of the second conductivity type and said base region of the second conductivity type.
A concentration of an impurity of the second conductivity type contained in said guard ring region is preferably higher than concentrations of impurities of the second conductivity type contained in said diode region of the second conductivity type and said base region of the second conductivity type.
It is preferable that concentrations of impurities of the second conductivity type contained in said guard ring region, said diode region and said base region of the second conductivity type, and said thin film region are progressively lower in this order.
As clearly described above, with the reverse conducting thyristor according to the present invention, it is possible to reduce the surface area size of the separation portion, and hence, to form the element in a small size.
Also, during fabrication of a plurality of such reverse conducting thyristors on a wafer, it is possible to reduce variations in insulation characteristics of the separation portion, and hence, to ensure that element characteristics are uniform.
Further, it is possible to prevent destruction of the element due to a leak current, and hence, to improve a production yield of the reverse conducting thyristors.